Polyimide films are well known polymers, and include any such polymers having an -imide linkage in the backbone or in the side chain of the polymer. These materials are commercially available under several different trade names, and are well known in the packaging of electronic circuits. These films are valued for their high thermal stability and excellent electrical properties. They have high heat resistance, good dielectric properties, are solvent-resistive, and can be applied by simple processing. While they are often used in molding and composite industries, the electronics industry uses them extensively as thin films.
Although polyimides have many attractive properties, it is very difficult to pattern films of polyimides, because they are not easily etched or dissolved away by the usual techniques. Thus, wet chemical processing is extremely difficult. In the art, reactive ion etching has been known as a technique for patterning polyimide films. However, this is a very slow process in which reactive ions, such as CF.sup.+.sub.3, react with the polyimide. This is a mechanical process in which these reactive ions are brought to the surface of the polyimide.
Since thin films of polyimide are extremely useful, it would be a distinct advantage to have a reliable technique for patterning these films. In particular, it is desirable to have a process which will provide high resolution patterning of films of polyimide. In the present invention, such a technique is described, where the polyimide is controllably photoetched by far UV radiation of wavelengths less than 220 nm, to produce patterns therein. The depth of the etching is at least 1000 .ANG., and can be entirely through the polyimide film.
Copending application Ser. No. 396,985 to Mayne-Banton and Srinivasan, assigned to the same assignee as the present application, describes a method for photoetching polyesters such as mylar (a trademark of DuPont) using far UV radiation of wavelengths less than 220 nm.
As is apparent to chemists, materials such as polyimide and mylar differ extensively, both in their structure and in their uses. For example, mylar is used in tapes and disks and as insulation for capacitors. Thus, mylar need not be patterned, but is often roughened to provide usefulness as a tape material. On the other hand, polyimides are used extensively in packaging and other applications where fine patterning is essential. It is quite pointless to merely provide roughening of the surface of a polyimide.
While far UV radiation of the same wavelength range has been used to photoetch mylar and polyimides, many of the effects seen are quite different. For example, far-UV radiation on mylar in a vacuum or nitrogen atmosphere will cause rapid etching of mylar when laser pulse radiation is used. However, only slow etching of mylar will occur when mercury lamps are used as a radiation source, such radiation also producing degradation of the film (cross-linking, hardening, and yellowing). In contrast with this, polyimides are nearly inert to exposure to radiation from mercury lamps in a vacuum or nitrogen atmosphere. They are also nearly inert to laser radiation under these same conditions.
Another distinction between polyesters and polyimide occurs when mercury lamp far-UV radiation is used for etching in an atmosphere of air. In air, mylar etched about 10-fold faster than in vacuum or nitrogen. On the other hand, polyimide exhibited very slow etching (about 1/6 that of mylar) at room temperatures in air. This rate increased 6-fold on heating the polyimide to 200.degree..
Still another distinction occurs when films of mylar and polyimides are subjected to far-UV radiation from an ArF excimer laser (193 nm) at intensities greater than 30 mJ/cm.sup.2. Mylar etches at the rate of 1500 .ANG./pulse when a 350 mJ/cm.sup.2 pulse is used. In contrast with this, polyimide etched at the rate of 750 .ANG. pulse only at comparable light intensity. An increase in temperature had no measurable effect on the polyimide etching.
As noted previously, the structure of polyesters is much different than that of polyimides. The mylar chain is very simple, as shown here: ##STR1##
This chain is simple enough that one break is sufficient to split it. Two breaks are enough to dislodge a small molecule. It is for this reason that mylar is highly responsive for far-UV radiation.
A representative polyimide is given by the following formula: ##STR2## All polyimides carry the functionality ##STR3## as an imide group. In the representative polyimide shown, the fused nature of the rings require that if a single bond is broken by the UV radiation, it most likely will not split the polyimide chain. Many more bond breaks are necessary to break up the chains sufficiently to dislodge a piece. Also, the polyimide is characterized by a higher percentage of carbon to hydrogen than is a polyester, such as mylar.
A general reason why polyimides are not, and would not be expected to be as affected by far-UV radiation as mylar may be the extraordinary thermal stability of the polyimides. A photon which breaks a bond in a polymer chain will also heat the polymer. This heating effect may promote further scissions in a polyester, but would be ineffective in the polyimide film.
A patent generally relating to a method for treating a thin surface of a plastic material is U.S. Pat. No. 4,247,496. In this patent, the plastic material is subject to an ultraviolet light treatment, after which it is stretched. The UV light has a wavelength ranging from 180 to 400 nm, and is emitted by sources such as mercury lamps, fluorescent lamps, xenon lamps, and carbon-arc lamps. The ultraviolet light treatment causes cracking in a surface layer (50-100 .ANG.) of the plastic. These cracks make stretching easier and leave a surface which contains widened cracks therein.
In U.S. Pat. No. 4,247,496, no photoetching is involved because it is important that only a thin surface layer be affected. That patent does not recognize that selected wavelengths of ultraviolet radiation can be used to efficiently photoetch polyimide. In fact, polyimides are not stretchable plastics and do not have linear chains which can be stretched and oriented.
Accordingly, it is a primary object of the present invention to provide a method for effectively etching polyimide.
It is another object of this invention to provide an improved method for etching polyimide, without requiring chemicals or subsequent development steps.
It is another object of the present invention to provide a technique for photoetching polyimides without requiring heating effects.
It is another object of the present invention to provide a method for etching polyimides in order to create high resolution patterns therein.
It is another object of the present invention to provide a technique for etching polyimide films using ultraviolet radiation of a specified wavelength for photodecomposing the polyimide.
It is a further object of this invention to provide a method for photoetching polyimides without the requirement for any liquid solvents, and to do so in a manner which does not rely on thermal effects.
It is a still further object of this invention to provide a method for etching patterns in polyimide films without modifying the bulk of the material, and without degrading or weathering the unetched portions of the polyimide film.
It is another object of this invention to provide a method for etching polyimide films by a technique which has etch rates that are much faster than previously used techniques for etching these films.